1. Technical Field
The present invention relates to a fixing device, and in particular to the fixing device employing an electromagnetic induction heating method to fix an unfixed image on a recording medium, and an image forming apparatus including such a fixing device.
2. Related Art
Conventionally, an image forming apparatus such as a copier, a printer, and the like, employs a fixing device employing electromagnetic induction, which is both fast and energy-efficient.
For example, JP-2006-350054-A discloses a fixing device employing the electromagnetic induction heating method, which includes a support roller as a heat roller to generate heat, a fixing support roller as a fixing roller, a fixing belt stretching around the support roller and the fixing support roller, an induction heater disposed opposite the support roller via the fixing belt, and a pressure roller pressing against the fixing support roller via the fixing belt. The induction heater is formed of a coil such as an excitation coil wound in the longitudinal direction and a core disposed opposite the coil. The fixing belt is configured to be heated at a portion opposite the induction heater. The thus-heated fixing belt heats to fix a toner image formed on a recording medium conveyed to a position opposite the fixing support roller and the pressure roller.
More specifically, when a high-frequency alternating current is supplied to the coil, an alternate magnetic field is formed around the coil, and an eddy current is generated near the surface of the support roller. When the eddy current is generated to the support roller as a heat roller, joule heat is generated due to electric resistance of the support roller itself. With its joule heat, the fixing belt wound around the support roller is heated. The fixing device employing the electromagnetic induction heating method as described above is known as a device with high-thermal conversion efficiency and less energy consumption, if compared to a conventional halogen heater, capable of increasing a surface temperature of the fixing belt up to a prescribed level in a short time because a heat generator used in the electromagnetic induction fixing device is directly heated by the method.
However, as to the fixing device employing the induction heating method, because the high-frequency alternating current is supplied to the coil, the temperature of the excitation coil rises from joule heat generated due to electrical resistance of the excitation coil. As a result, there is a concern that UL Standards for Safety are not observed due to the increase in the temperature of the coil or the excitation coil is broken.
As an approach to solve the above problem, a technology is known in which the excitation coil is cooled using a cooling fan. For example, JP-2000-105516-A discloses a fixing device including an insulation member between the excitation coil and the fixing roller so that the cooling efficiency of the cooling fan is improved. However, in the fixing device employing the induction heating method the coil temperature has a tendency to rise more quickly than in the conventional device when, for example, used in a high-speed printer capable of printing speeds of more than 60 cpm (copies per minute) because of an increase in the continued printing time and a high power input for the elevated temperature for short-time start-up. Currently, there is no other way to increase the power of the cooling fan in order to increase cooling efficiency, which causes problems of cost increase, increased energy consumption, increased noise, and the like.